[llvm-branch-commits] [llvm] release/23.x: [AArch64][FastISel] Avoid TBZ with SLH more thoroughly. (#217329) (PR #217993)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/217993

Backport eb2e28ad666b537837f7ee0ca88f6f66c12d2aeb

Requested by: @ahmedbougacha

>From 0afeb2ca2fafd1c39c7ec46b0567f43e40083acb Mon Sep 17 00:00:00 2001
From: Ahmed Bougacha <ahmed at bougacha.org>
Date: Fri, 21 Aug 2026 14:10:25 +0200
Subject: [PATCH] [AArch64][FastISel] Avoid TBZ with SLH more thoroughly.
 (#217329)

With SpeculativeLoadHardening, we already avoid using cond-branch
instructions that don't modify NZCV (CBZ, TBZ), since we rely on NZCV to
propagate the speculative taint mask with CSEL later on.

However, in one case in FastISel, we still emit TBZ, ignoring SLH. Make
it honor the SLH attribute like we do elsewhere.

(cherry picked from commit eb2e28ad666b537837f7ee0ca88f6f66c12d2aeb)
---
 llvm/lib/Target/AArch64/AArch64FastISel.cpp        |  5 +++++
 llvm/test/CodeGen/AArch64/speculation-hardening.ll | 14 ++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/llvm/lib/Target/AArch64/AArch64FastISel.cpp b/llvm/lib/Target/AArch64/AArch64FastISel.cpp
index 6c218de2c43c3..b6deb703893fd 100644
--- a/llvm/lib/Target/AArch64/AArch64FastISel.cpp
+++ b/llvm/lib/Target/AArch64/AArch64FastISel.cpp
@@ -2483,6 +2483,11 @@ bool AArch64FastISel::selectBranch(const Instruction *I) {
     return false;
 
   // i1 conditions come as i32 values, test the lowest bit with tb(n)z.
+  // However, that's not allowed with SLH.
+  if (FuncInfo.MF->getFunction().hasFnAttribute(
+          Attribute::SpeculativeLoadHardening))
+    return false;
+
   unsigned Opcode = AArch64::TBNZW;
   if (FuncInfo.MBB->isLayoutSuccessor(TBB)) {
     std::swap(TBB, FBB);
diff --git a/llvm/test/CodeGen/AArch64/speculation-hardening.ll b/llvm/test/CodeGen/AArch64/speculation-hardening.ll
index cb445b47f907f..49aa549cb276a 100644
--- a/llvm/test/CodeGen/AArch64/speculation-hardening.ll
+++ b/llvm/test/CodeGen/AArch64/speculation-hardening.ll
@@ -99,6 +99,20 @@ else:
   ret i32 %6
 }
 
+define i32 @test_branch_zero_i1(i32 %a0, i32 %a1, i32 %a2) SLHATTR {
+; CHECK-LABEL: test_branch_zero_i1
+  %tmp0 = trunc i32 %a2 to i1
+  br i1 %tmp0, label %then, label %else
+;SLH-NOT:   tb{{n?}}z
+;NOSLH:     tb{{n?}}z
+then:
+  %tmp1 = sdiv i32 5, %a1
+  ret i32 %tmp1
+else:
+  %tmp2 = sdiv i32 %a1, %a0
+  ret i32 %tmp2
+}
+
 define i32 @landingpad(i32 %l0, i32 %l1) SLHATTR personality ptr @__gxx_personality_v0 {
 ; CHECK-LABEL: landingpad
 entry: